Repair of DNA damage induced by ionizing radiation and benzo[a]pyrene in mammalian cells

Interspecies Differences in the Removal of DNA Adducts

Numerous physical and chemical agents damage cellular DNA in vivo. Such damage has been associated with various biochemical, physiological, and pathological dysfunctions including: alterations in gene expression, cell death, mutation, birth defects, cancer, and aging. Cells and organisms unable to prevent the induction of DNA damage or to repair such damage once it is induced, are predisposed to one or more of these pathologies. Over the course of evolution, living systems have developed various mechanisms to cope with such damage including enzymatic repair, information redundancy and, in extreme situations, cellular replacement. Enzymatic repair can be divided into two general categories: prereplication and post-replication repair. Each of these categories includes many “repair” systems which differ with the size of the repaired region, the nature of the enzymes involved in the repair process, the type of agent inducing the repair process or form of lesion removed. Over the last decade, numerous methods have been developed to measure DNA damage, both directly as well as indirectly, but few studies exist comparing the results of these methods with one another. Little is known as to whether these methods are measuring the same or different endpoints. Interspecies, intertissue, and interorgan comparisons can only be made when comparable techniques have been utilized. From such studies, it is now apparent that significant differences in DNA repair exist among species, within species, and between organs. Further, it is now a reasonable speculation that such differences may, in part, account for differences in organ susceptibility and risk per cell per unit time for spontaneous malignant transformation observed between species.

Ah locus-associated differences in induction of sister-chromatid exchanges and in DNA adducts by benzo[a]pyrene in mice

The effect of alleles of the Ah locus on the induction of sister-chromatid exchanges (SCE) was studied in C57Bl/6 and in DBA/2 mice treated twice intragastrically with benzo[a]pyrene (BP, 100 or 10 mg/kg b.w.). To measure the changes in the frequency of SCE, 2 protocols were used: in vivo in bone marrow cells after implantation of 5-bromodeoxyuridine (BrdU) tablets and in vivo/in vitro in spleen lymphocytes cultured with BrdU. On day 5 mice were killed and SCEs estimated in bone marrow cells. BP-DNA adducts in bone marrow and spleen were analyzed on day 5 after the same exposure to BP. In the spleen lymphocytes SCE frequencies were analyzed after an additional 48 h of culture. We found that at both doses of BP, the number of SCEs and BP-DNA adducts in bone marrow and in spleen cells was significantly higher in aryl hydrocarbon hydroxylase (AHH)-non-inducible (DBA/2) mice than in AHH-inducible (C57BL/6) mice. Only marginal induction of SCE was noted after the high dose of BP in C57BL/6 mice in bone marrow in vivo, whereas a highly significant increase in the frequency of SCEs was found in splenocytes in the in vivo/in vitro test. The spleen cells contained larger amounts of BP-DNA adducts and demonstrated higher absolute levels of SCEs than bone marrow cells. The sensitivity of both the in vivo/in vitro and the in vivo SCE test is high enough for assessment of Ah locus-linked differences in BP genotoxicity in mice at the prolonged time between treatment and cell preparation. The present data confirm the influence of inducibility of AHH in the intestine on the genotoxicity of BP to distal tissues after oral exposure to BP.

A technique for the measurement of breakage and repair of DNA alkylated in vivo

The alkaline elution technique has been adapted for use in the assessment of DNA damage induced in the livers and lungs of mice after administration of an alkylating agent, methylemthanesulfonat (MMS). At 4 h after administration of MMS, damage ot DNA was readily demonstrable; the damage was repaired in liver by 24 h. The lung, particularly of the A/J mouse, exhibited an increased alkaline elution rate when compared to C57BL/6J, and repair was not entirely complete (as judged from the rate of alkaline elution of DNA) by 24 h. The rate of elution was dependent upon temperature. It is believed that this adaptation should have great utility in examining DNA repair in vivo.